This invention relates to a method and apparatus for monitoring the operation of a system and, in particular, to a monitor capable of recognizing the difference between a normal interruption in service and an interruption of greater duration that might result in a total failure in the operation of the system.
Many systems that are in use today are adapted to maintain a constant or uniform environmental level within specific system boundaries. Failure of the system usually results in some type of injury occurring to either the equipment or goods stored in the environment. An example of such a system is a food freezer as typically found in most of today's markets. As can be seen, if the temperature within the freezer is allowed to rise beyond some predetermined level for a prolonged period of time, the goods stored in the freezer can thaw and possibly spoil. However, most freezers in use today are adapted to automatically defrost twice daily with each cycle lasting approximately sixty minutes. It is therefore difficult to determine when the freezer is experiencing a normal defrost cycle and when it is experiencing a failure related interruption in service.
It is possible to overcome the difficulties associated with defrost cycles and the like by providing a temperature monitoring system that has a time delay circuit capable of temporarily disabling an associated warning device for a period of time equal to or slightly greater than the defrost period upon the detection of a rise in temperature. Accordingly, the alarm is not actuated until after the monitoring system has waited for a period of time at least equal to the duration of a defrost cycle. Although the time delay feature prevents the generation of false alarm signals when the equipment is experiencing a normal interruption in service, it can introduce other problems which are also difficult to deal with. One problem encountered in this regard lies in the fact that the user has no way of determining if the monitor has failed or if it is simply waiting for a delay period to end. Oftentimes, the inoperability of the monitor can go undetected for long periods of time during which the system being monitored is unprotected. A system failure during this time thus would not trigger the alarm and, as a consequence, the user could very well suffer a loss. Similarly, at the time of installation, the installer generally has no way of knowing if either the monitoring circuit or the alarm are operating correctly. To properly check the apparatus, the installer must minimally follow the device through one complete normal delay cycle. In any event, a good deal of time can be consumed before it is determined that the monitor is operating properly.